In reality, this does not happen that way. If a path already exists, you can pay to use the same duct (unless it's full) to install your own fibers. At least, it works this way in France.
ETA from my ISP to actually get any of those lines into my apartment is still 2028.
So if one company is doing the east part of the town, and another company is doing the west, at some point they could leverage the infrastructure of each other.
I don't think so. German open access law says if you're in a "market dominating" position, you have to share access to what you have. I'm at least unaware of a law that allows forcing telcos to build more than they want. (And even if there were, it'd be balanced by them deciding to just not build at all.)
Worse, two companies are chasing each other, so you have the ground being opened, closed, and within half a year the other company does the same. Even worse, they let the fiber cables stick out of the ground until the house subscribes to them, so a lot of houses have 1 or 2 ugly orange cables sticking out of the ground for years near the front door.